Thy-1 is a membrane glycoprotein expressed predominantly on T lymphoyctes and neuronal cells. Because of its structural homology to immunoglobulin domains, it has been proposed that Thy-1 may function as a receptor on T cells. Indeed, several studies have demonstrated that antibodies directed against Thy-1 can stimulate T cells to proliferate and produce lymphokines. In contrast to other membrane proteins involved in regulation of the immune system, Thy-1 appears to be anchored to the membrane via a glycophospholipid which replaces a 31 amino acid segment at the carboxyl terminus of newly synthesized Thy-1. Mutant and chimeric Thy-1 genes will be constructed and used in transient and stable transfection systems to investigate the mechanism of this conversion and the role of the glycolipid anchor in membrane "trafficking" and T cell activation.